The present invention relates to ultrasonics, and more particularly to all or portions of an ultrasonic system which includes coupling signals between transducer and an object or body of material through which those signals are to propagate. Particular applications may involve coupling into a gas at high temperature where some form of isolation, such as a buffer rod, is required.
Buffer rods have been used in ultrasonics for over fifty years to separate a transducer crystal from media under investigation that are at very high temperature. This is analogous to tending a red-hot fire with a long steel poker so as to not burn one's hands.
In ultrasonic measurements, it is important that the buffer not corrupt the signals of interest. Accordingly, much effort over the years has gone into avoiding the sidewall echoes generated by mode conversion. Such mode conversion occurs when longitudinal waves strike the wall near grazing incidence, generating shear waves, which in turn reflect multiple times diagonally across the rod. Each reflection generates a delayed replica of the original longitudinal pulse. Probably the most commonly-used way to avoid sidewall echoes is to thread the buffer. This method is often adequate, but has the disadvantage that it is quite lossy . For example, a solid steel buffer rod of 25 mm diameter and length of about 30 cm experiences a beam spread loss of about 20 dB, for a 500 kHz signal, assuming the longitudinal wave starts as a plane wave across the entire diameter of the rod.
Other known methods for preventing beam spread or diffraction loss in solid buffer rods include
(i) the use of shear waves rather than longitudinal waves, as described in U.S. Pat. No. 3,477,278 of L. Lynnworth, and PA1 (ii) the use of a buffer rod in which the outer portion has a higher sound speed than the core, as described in U.S. Pat. No. 5,241,287 of Jen.
Furthermore, if hollow buffers are considered, one can avoid most of the diffraction loss and also avoid sidewall echoes. But in certain applications, it is not easy to correctly eliminate errors due to uncertainties in the time of travel down a hollow tube, in which the sound-conducting fluid may have a temperature or compositional gradient. Hollow tubes also run the risk of becoming filled with residues or condensate, which loads the walls and significantly alters their propagation properties. Also, at high flow velocities they resonate and create strong acoustic interferences.
In 1966, I. L. Gelles described non-dispersive operation of individual or bundled glass fibers to form flexible ultrasonic delay lines. These bundles, formed of fiber optic cable encased in a loose plastic sheath, had their fibers all fused or joined together by epoxy to form a practical termination. However, Gelles found high attenuation even in a very short distance of propagation through the bonded region, reporting an attenuation of "roughly greater than 10 dB/mm of coated-fiber length." (I. L. Gelles, Optical-Fiber Ultrasonic Delay Lines, J. Acoust. Soc. Am., 39 (6), pp. 1111-1119 (1966)). With this construction, Gelles was able to adjust the bond thickness to work better for a particular frequency or to minimize an undesired or spurious pulse, and he suggested that that construction would have usefulness for pick-off points, re-entrant delay lines and particular devices such as fiber-based acoustic modulators. However, to applicant's knowledge, the acoustical use of optical fibers has not found application to transmission link or buffer constructions in the subsequent decades.
Thus, there remain problems in delivering or recovering well-defined acoustic signals when the process or measurement environment requires that the transducer be spatially remote. These problems may be particularly daunting when the process involves a gas at high temperature and high pressure, so that multiple considerations of physical isolation or containment, signal strength, and acoustic path impedance discontinuities all affect performance. For example, the seemingly simple requirement of measuring gas flow velocity accurately within .+-.1% over a wide flow range, for a low molecular weight gas, starting near or at atmospheric pressure and building up to 200 bar continuous, and for gas temperatures ranging up to 200.degree. C. in normal operation and 450.degree. C. in upset conditions, actually imposes a long list of requirements on the ultrasonic measuring system.
A practical system must accommodate considerations of cost, flexibility, signal isolation, and useful frequency range, as well as factors relating to calibration, maintainability and compatibility with existing equipment.
Accordingly, it would be desirable to provide an improved ultrasonic system.